TTY is a device that enables hearing impaired or deaf individuals, herein collectively referred to as hearing impaired individuals, to communicate via a telephone using text or characters. Original TTY machines were acoustic couplers that held the handset receiver of a telephone and were used with the PSTN. To continue providing this service in the United States of America (USA), the Federal Communications Commission of the USA requires service providers and cell phone manufacturers to provide accessibility for hearing impaired individuals in cellular telephone systems.
Cell phones use a cellular text telephone (CTM) modem to provide TTY service for hearing impaired individuals. A CTM modem has a transmitter and a receiver that allows the reliable transmission of text using a speech channel of cellular telephone systems or PSTN networks. The CTM modem can be external to the cell phone or included within a cell phone.